This patch makes the tests (including the test driver) run entirely
inside Vagrant, which avoids calling the very slow `vagrant` driver many
times for running the tests. On my machine, `./test` runs in 22 seconds,
down from hundreds of seconds prior to this patch.
This also has the nice side effect of matching how the Travis CI tests
were run, so there's no need for a separate `test_travis` anymore.
Previously, clean read the defaults once, and then it updated the
setting for each entry it read. This resulted in the defaults being
clobbered and then not being respected for subsequent entries. This
patch fixes the issue by re-reading the defaults before processing each
The other plugins (link, shell) do not have this problem.
This patch makes the launcher script prefer `python`, when present, over
`python3`. This way, the launcher uses the user's preferred `python`
(which is often set up as a symbolic link to a particular python2.x or
python3.x), when available.